The present invention relates to an exhaust gas purification system for an internal combustion engine, particularly of the kind having a NOx trap catalyst that traps NOx in an exhaust gas when an exhaust air/fuel ratio is lean and releases the trapped NOx when the exhaust air/fuel ratio is rich, thereby purifying the exhaust gas. The present invention further relates to an exhaust gas purification method for an internal combustion engine.
In the diesel engine, when injection of fuel into a cylinder is started, injected fuel is vaporized and formed into a mixture. The mixture that exists within the cylinder when the temperature and pressure within the cylinder reach predetermined values performs an initial combustion (premix combustion). By this combustion, the temperature and pressure within the cylinder become high, so that after the initial combustion the injected fuel is vaporized and burnt (diffusion combustion) simultaneously with the injection. Since the period (ignition delay period) from start of fuel injection to occurrence of the premix combustion is generally short with respect to the entire combustion period, the diffusion combustion is a main combustion.
In contrast to this, Japanese Patent 2864896 discloses a technique for holding down the combustion temperature to low by increasing the EGR rate considerably, while retarding the fuel injection timing to a point after the top dead center thereby making an ignition delay period considerably longer and allowing all the quantity of fuel to be injected within the ignition delay period for allowing a premix combustion to serve as a main combustion, thereby reducing the NOx and smoke in the exhaust gas.
In the meantime, a combustion condition wherein a main combustion is a diffusion combustion (this is herein referred to simply as diffusion combustion or diffusion combustion mode) and a combustion mode wherein a main combustion is a premix combustion (this is herein referred to simply as premix combustion or premix combustion mode) are generally switched from one to another depending upon a driving condition (for example, in FIG. 8, a zone with EGR corresponds to a premix combustion zone and a zone without EGR corresponds to a diffusion combustion zone).
On the other hand, Japanese Patent 3079933 discloses that in case an exhaust passage is provided with a NOx trap catalyst and at the time for purifying the trapped NOx, the combustion mode of the diesel engine is switched from the diffusion combustion mode to the premix combustion mode thereby lowering the excess air rate and reducing and purifying the trapped NOx.
The technique disclosed by Japanese Patent 3079933 is adapted to execute a rich spike control during the premix combustion at the time of purification of NOx trapped in the NOx trap catalyst. However, the technique has a problem that even when the exhaust air/fuel ratio is made rich when the vehicle is in a low-load running condition, a sufficient exhaust purification effect by the rich spike control cannot be attained since the exhaust gas temperature remains low and the low exhaust gas temperature causes the NOx purifying efficiency to be maintained low, i.e., the catalyst is not sufficiently activated.
It is accordingly an object of the present invention to provide an exhaust gas purification system for an internal combustion engine that can attain an improved exhaust gas purifying ability by executing purification of trapped NOx of the NOx trap catalyst depending upon an activated condition of the NOx trap catalyst.
It is a further object of the present invention to provide an exhaust gas purification system that is executed by the exhaust gas purification system of the foregoing character.
According to an aspect of the present invention, there is provided an exhaust gas purification system for an internal combustion engine including a NOx trap catalyst disposed in an exhaust passage of the engine for trapping NOx in an inflow exhaust gas when an air/fuel ratio of the exhaust gas is lean and releasing trapped NOx when the air/fuel ratio of the exhaust gas is rich, and a control unit for controlling the NOx trap catalyst in accordance with an activated condition of the NOx trap catalyst, the control unit comprising a purification time determining section for determining whether it is the time for purifying NOx trapped by the NOx trap catalyst, a catalyst activation determining section for determining whether the Nox trap catalyst is activated, a combustion mode switching section for switching a combustion mode of the engine between a diffusion combustion mode and a premix combustion mode in accordance with an operating condition of the engine, and an air/fuel ratio control section for controlling an exhaust air/fuel ratio so that the exhaust air/fuel ratio is made rich in the premix combustion mode when it is the time for purifying trapped NOx of the NOx trap catalyst and the NOx trap catalyst is activated and that the exhaust air/fuel ratio is made rich in the diffusion combustion mode when it is the time for purifying trapped NOx of the Nox trap catalyst and the Nox trap catalyst is not activate.
According to another aspect of the present invention, there is provided an exhaust gas purification method for an internal combustion engine having a NOx trap catalyst disposed in an exhaust passage for trapping NOx in an inflow exhaust gas when an air/fuel ratio of the exhaust gas is lean and releasing trapped NOx when the air/fuel ratio of the exhaust gas is rich, the exhaust gas purification method comprising determining whether it is the time for purifying NOx trapped by the NOx trap catalyst, determining whether the Nox trap catalyst is activated, switching a combustion mode of the engine between a diffusion combustion mode and a premix combustion mode in accordance with an operating condition of the engine, and controlling an exhaust air/fuel ratio so that the exhaust air/fuel ratio is made rich in the premix combustion mode when it is the time for purifying trapped NOx of the NOx trap catalyst and the NOx trap catalyst is activated and that the exhaust air/fuel ratio is made rich in the diffusion combustion mode when it is the time for purifying trapped NOx of the Nox trap catalyst and the Nox trap catalyst is not activated.
According to a further aspect of the present invention, there is provided an exhaust gas purification system for an internal combustion engine comprising a NOx trap catalyst disposed in an exhaust passage of the engine for trapping NOx in an inflow exhaust gas when an air/fuel ratio of the exhaust gas is lean and releasing trapped NOx when the air/fuel ratio of the exhaust gas is rich, purification time determining means for determining whether it is the time for purifying NOx trapped by the NOx trap catalyst, catalyst activation determining means for determining whether the Nox trap catalyst is activated, combustion mode switching means for switching a combustion mode of the engine between a diffusion combustion mode and a premix combustion mode, and air/fuel ratio control means for controlling so that the exhaust air/fuel ratio is made rich in the premix combustion mode when it is the time for purifying trapped NOx of the NOx trap catalyst and the NOx trap catalyst is activated and that the exhaust air/fuel ratio is made rich in the diffusion combustion mode when it is the time for purifying trapped NOx of the Nox trap catalyst and the Nox trap catalyst is not activated.